Aromatic and Alkene Monooxygenase Hydroxylase, subunit B, ferritin-like diiron-binding domainAromatic and Alkene Monooxygenase Hydroxylases, subunit B (AAMH_B). Subunit B (beta) of the soluble hydroxylase of multicomponent, aromatic and alkene monooxygenases are members of a superfamily of ferritin-like iron-storage proteins. AAMH exists as a hexamer (an alpha2-beta2-gamma2 homodimer) with each alpha-subunit housing one nonheme diiron center embedded in a four-helix bundle. The N-terminal domain of the alpha- and noncatalytic beta-subunits possess nearly identical folds; the beta-subunit lacks the C-terminal domain found in the alpha-subunit. Methane monooxygenase is a multicomponent enzyme found in methanotrophic bacteria that catalyzes the hydroxylation of methane and higher alkenes (as large as octane). Phenol monooxygenase, found in a diverse group of bacteria, catalyses the hydroxylation of phenol, chloro- and methyl-phenol and naphthol. Both enzyme systems consist of three components: the hydroxylase, a coupling protein and a reductase. In the MMO hydroxylase, dioxygen and substrate interact with the diiron center in a hydrophobic cavity at the active site. The reductase component and protein coupling factor provide electrons from NADH for reducing the oxidized binuclear iron-oxo cluster to its reduced form. Reaction with dioxygen produces a peroxy-bridged complex and dehydration leads to the formation of complex Q, which is thought to be the oxygenating species that carries out the insertion of an oxygen atom into a C-H bond of the substrate. The toluene monooxygenase systems, toluene 2-, 3-, and 4-monooxygenase, are similar to MMO but with an additional component, a Rieske-type ferredoxin. The alkene monooxygenase from Xanthobacter strain Py2 is closely related to aromatic monooxygenases and catalyzes aromatic monohydroxylation of benzene, toluene, and phenol. Alkane omega-hydroxylase (AlkB) and xylene monooxygenase are members of a distinct class of integral membrane diiron proteins and are not included in this CD.